A: 13.56 MHz is one of several I.S.M. frequencies mandated by the FCC. These are frequencies set aside for industrial, scientific, and medical purposes and have relaxed emissions limitations because they are so positioned in the radio wave spectrum as to not interfere with safety, emergency, and military communications. It just happens that this particular ISM frequency has the most plasma research historical data.
A: A linear amplifier is one in which the output current is directly proportional to the voltage input. Linear amplifiers do not distort the signal or cause hamonics to be introduced. A class A amplifier is an example of a linear amplifier. A nonlinear amplifier is one that does not operate over the linear operating region of the transistor or tube. Instead, the device is used primarily as a switch to turn the current off and on. Nonlinear amplifiers generate pulses or square waves at the frequency of operation.
A: This generally means that the generator is in "reflected foldback." The generator is limiting its forward power output to protect itself against excessive reflected power. With our generators, this foldback occurs when the reflected power reaches about 20% of the full rated power of the generator. This condition is seldom the fault of the RF generator. The problem might lie in the coax, the matching network, or a short of open in the RF chain after the match. In plasma applications, any condition that prevents the striking of plasma will result in high reflected power.
A: Under these conditions, some energy is being delivered to the load, but the impedance matching mechanism of the system is not able to match the load well enough. If the generator is fixed frequency, then the fault lies elsewhere. If the generator adjusts its frequency to accomplish impedance matching, then this may be suspect, but so might the impedance matching network that works with the generator. Other causes include chamber conditions resulting in plasma impedance outside the range of the matching network.
A: First, check to see if AC Switch on the back of the generator is on. Next, verify that there is AC power to the generator (verify AC power with a voltage meter). If these conditions have been verified and there are still no lights lit and no display present, the generator will need to be repaired.
Test Equipment for Troubleshooting:
To properly test your supply, you need a standard portable digital multimeter (DMM), a power-SWR meter, and a dummy load.
Multimeter: It measures AC/DC voltages, amperage, and ohm resistance of pots and pickups. It is used to measure line voltage and, when set as an ohm meter, it can be used to check the coax cable.
Power-SWR Meter: Our generators have built-in power meters that show forward and reverse power, but for setting up and testing power supplies, an external meter is useful.
Dummy Load: Dummy loads are necessary for troubleshooting the generator at medium and high power levels. Commercial dummy loads are available with a 50-ohm resistive, noninductive impedance, with power levels ranging from a few hundred to many thousands of watts. The dummy load is usually connected to the output of the RF generator with a short 50-ohm coaxial cable and allows you to put maximum power into a standard load while making tests on the generator. When testing RF generators using impedance-matching networks, a dummy load that more accurately represents the load in the system is highly desirable.
Additional equipment:
Oscilloscope: This is used to observe the signals from the crystal oscillator through the various driver, buffer, and power amplifier stages. The scope will show any signal distortion, noise, or other characteristics that cannot be detected with a voltmeter.
Frequency Counter: The primary use of this piece of equipment is to check the operational frequency of the RF generator and to verify that the crystal oscillator is on frequency. RF generators are regulated by the FCC and must operate on specific frequencies within a given tolerance range. If the actual frequency deviates too much, adjustments may be necessary. Frequency adjustments are madeonly after the equipment has warmed up for at least an hour—it’s important that the generator be at a stable normal temperature before adjusting the frequency.
Spectrum Analyzer: This Oscilloscope-like equipment is useful in diagnosing unusual troubles in a generator that may be related to distortion and harmonics or spurious signals. Most spectrum analyzers are used in RF communications systems where they can show the sidebands developed during modulation as well as harmonic distortion and for detecting and isolating interference.
RFI-EMI Tester: This test equipment is a highly sensitive radio receiver designed to detect stray RF radiation, including harmonics and intermodulation products. The more sophisticated test sets incorporate a spectrum analyzer.
Field Strength Meter: This is a simple radio receiver used to measure the amount of signal radiated from an RF source.
Troubleshooting Safety Tips:
Troubleshooting First Steps:
WARNING: Equipment must be installed, operated and serviced only by trained, qualified persons.
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